Radio transmitter



Feb. 14, 1939. y G, w. FYLER 2,147,486

\ I RADIO TRANSMITTER Original Filed Dec. 6, 1935 SIG/VAL AMPL/F/ER ISAttorney.

5- PA 0,40 GGF e Wjjlla, `o l mol/rc v V bg 8 Patented F eb. 14, 1939PATENT OFFICE RADIO TRANSMITTER George Fyler, Stratford, Conn., assignorto General Electric Company, a corporation of New York Application'December 6, 1935, serial No. 53,172

Renewed January 11, 1938` 9 Claims. (o1. 178-44) My invention relates toradio transmitting apparatus, and more particularly to apparatus of theshort wave high power type.

In short wave transmitters, as for example, television transmitters, itis necessary to modulate the radio frequency carrier Wave with signalWaves having frequencies which vary over an exceedingly wide range.Thus, in television transmission the signal currents may contain fre- 10quencies extending from a low audio frequency of cycles to anexceedingly high radio frequency of 2,000,000 cycles. If satisfactoryoperation of a transmitter of this type is to be obtained, it isnecessary that the carrier wave be modulated to the correct degree byall of the signal current frequencies within this broad range.

In other words, the frequency-modulation characteristic should be flat.Such modulation can only be obtained if a coupling impedance be pro- 20vided between the modulator amplifier stage, and the output circuit ofthe carrier power amplifier stage having an impedance characteristicsuch that the impedance to currents of all frequencies within the rangeis above a predetermined value.

5 It has been found that the impedance of an iron core coupling reactorordinarily employed in high power work drops off to a value considerablylower than the predetermined value necessary to preserve the'desireddegree of modulation at so the high frequencies in the upper portion ofthe Wide range specified. This is due to the fact that the type ofreactor specified has a high distributed capacity which, in the upperfrequency ranges, causes the reactor to function as a condenser having alow capacity reactance. The effect of the low impedance to the highfrequency components of the signal current is to reduce the degreel ofmodulation of the carrier at such high frequenciesand introduceundesired distortion into the modulated carrier transmitted.

It is an object of my invention to obviate the above difficulties byproviding an improved modulator stage output circuit coupling impedancenetwork suitable for use in a high power radio transmitter which offersan impedance above a predetermined value to currents of all frequencieswithin an exceedingly wide range.

In accordance with my invention the above stated object is realized byproviding a modulator coupling impedance network which includes an aircore reactor having a very low distributed capacity shunted by aresistance and connected in series with an iron core reactor having ahigh distributed capacity; the impedance values of the componentelements of the network being so selected that the over-all impedancebetween the terminals of the network is maintained at a value above thatnecessary to produce satisfactory modulation of the carrier wave bysignal currents having component frequencies extending throughout thewide frequency range specifled above.

Accordingly it is a further object of my invention to provide animproved coupling impedance network for coupling the output circuit of atransmitter modulator amplifier stage to the output circuit of thecarrier wave power amplifier stage which comprises the above-describedarrangement of impedance elements.

The novel features which I believe to be characteristic of my inventionare set forth with particularity in the appended claims. My inventionitself, however, both as to its organization and method of operation,together with further objects and advantages thereof will best beunderstood by reference to the following description taken in connectionwith the accompanying drawing in which Fig. l illustrates my inventionembodied in a short Wave high power radio transmitter adapted totransmit television signals; Fig. 2 illustrates the impedancecharacteristic of one of the elements of the coupling impedance networkillustrated in Fig. l, and Fig. 3 represents the impedancecharacteristic curve of my improved modulator coupling impedancenetwork.

Referring to Fig. l of the drawing, I have illustrated my invention asapplied to a television transmitter of conventional desi-gn. As shown,the transmitter comprises a radio frequency carrier wave generator Iconnected to supply its output through a driver stage 2, a push-pullclass C connected power amplifier system 3 and the windings 4 and 5 of acoupling transformer 6 to the conductors 'I and 8 of an antennatransmission line system. The carrier wave output from the poweramplifier is modulated in accordance with the signal oscillationsproduced by a signal current source 9. These signal oscillations areamplified by the cascade class A connected amplifiers I0, II and I2 andthe constant current modulator amplifier stage I3 and are impressed onthe output circuit of the power amplifier 3 through a modulator couplingimpedance network I4. 'I'his network comprises an air core inductor I5having a very low distributed capacity shunted by a resistance I6 andconnected in series with an iron core reactor I'I having an inherentlyhigh distributed capacity. One terminal of the network I4 is tapped tothe midpoint of the winding 4 and the other terminal is connected to thepositive side of a high voltage source (not shown) which is provided tosupply energy to the respective anode circuits of the transmittersystem.

In order to neutralize the effect of stray capacity in the carrier poweramplifier on the modulator stage an inductance coil I8 is provided whichis connected in the modulator stage plate circuit in the mannerillustrated. It will be understood that such stray capacity is caused bythe distributed capacity of the coils 4 and 5 and-also by the effect ofthe neutralizing condensers included in the power amplifier-circuit.This stray j capacity has been found to be small as compared to thedistributed capacity of the reactor Il and accordingly the value ofinductance of coil i8 may be small.

In television transmitters wherein thesource of signal oscillations 9comprises a photo-electrical cell system, a sourceof light,anda-scanning disk, all suitably arranged `to scan the. object Vto betelevised .with a spot of light, the signal current frequencies whichare produced by the source and which are amplified and impressed on theplate circuit ofthe power amplifier 3 cover an exceedingly widefrequency band which may extend from a low frequency of 20 cycles to avery high frequency of 2,000,000 cycles. If satisfactory operation ofthe transmitter is Yto be obtained, all of the component frequencieswithin this range must be amplified by the same relative amount andimpressed on the output circuit of the power amplifier 3 to produce thesame relative degree of modulation. If certain of the frequencies arediscrimin'ated against, as for example, the frequencies in the highportion of the range, frequency distortion results which produces ablurred image or lack of definition when the signals are reproduced in areceiving system.

One problem involved in the construction of a signal current circuitcapable of meeting the above requirement is that of providing a couplingimpedance for impressing the output from the modulatorstage I3 on theoutput circuit of the power amplifier 3 which offers a high impedancehaving a value above a predetermined minimum value to currents of allfrequencies within the band. Thus, it is known that the value of themodulator coupling impedance must be at least two times the impedance-ofthe load into which the coupling impedance operates at allsignalfrequencies within the range, if undistorted modulation is to beobtained.

t has been found that when an iron core reactor-of the type usuallyemployed in high power installations asa modulator coupling impedance isused, the above relation vbetween the coupling impedance and themodulated load impedance is not preserved at the high signal frequenciesdue tothe relatively high distributed capacity of the reactor. Thisdistributed capacity causes the reactor to act as a low impedancecapacitive reactance at high frequencies above a predetermined value. Ithas further beenfound that as the frequency is raised'above thispredetermined value the capacity reactance gradually `approaches zero,which of course means that the desired relation between the couplingimpedance and the modulated load impedance is destroyed and unequalizedmodulation of the amplified signal frequencies results.

The above noted phenomena is clearly illustrated in Fig. 2 wherein thecurves A and B, which were plotted from test data taken on an installediron core coupling reactor, show that the over-all impedance of thereactor decreased gradually to- Ward zero as the frequency of theapplied current was raised above 20 kilocycles. In the installationtested the minimum tolerable over-all impedance of the reactor forsatisfactory modulation was 10,000 ohms, corresponding to a loadimpedance of 5,000 ohms. Curve B, which is an extension of curve A on asmaller scale, shows that above 100,000 cycles the impedance of thereactor approached and became less than the load impedance-ofthe loadinto which the reactor operated. The tests determined the capacitance ofthe reactor as being 160 micromicrofarads at a frequency of kilocycleswhich value gradually decreased to approximately 100 micromicrofarads ata'frequency of 1000 kilocycles.

'In accordance with my invention these difficultiesare avoided byproviding in series with reactor I'I the air core inductor I5 having avery low distributed capacity -and shunted by the resistance I6. Theimpedance network thus formed may be resolved into lan equivalent seriesnetwork, as indicated by the dotted lines, which network is anapproximaterepresentation of the circuit inthe high frequency portion ofthe frequency range if the capacitive reactance of the coil I5 beneglected. The impedance 'values of the respective elements I5 and I6are of course determined bythe value of the modulated load impedanceinto which vthe coupling impedance operates. Thus, if the value of themodulated load impedance, and the impedance characteristic of thereactor II,be known,'the impedance'values of the elements I5 and i6 maybe readily determined. The arbitrary lower impedance value for thecoupling impedance network may be placed at two times the load impedancein accordance with the requirements described above. At this arbitrarilydetermined minimum the impedance of the reactor is capacitive and fromthe frequency-impedance characteristic curve ofthe reactor theparticular frequency at which'this lower limit is reached maybedetermined. Using the frequency value as thus determined, the value ofinductance of the element I5 necessary to introduce an inductivereactance I5 into the equivalent series circuit which exactlyneutralizes the capacitive reactance I'I of the reactor I'I, andthevalue of resistance l5 which produces an equivalent resistance IB equalto the required value of atleast two times the load impedance maybecalculated in a well known manner. It can be shown mathematically thatthe inductor I5 should have at the particular determined frel quency areactance valueequal to the value of the resistance I6 and that theimpedance value of each of the elements i5 and I6 should be at least twotimes the lower arbitrary impedance limit necessary to preserveequalized modulation over the entire frequency band.

Following the determination of the impedance values vof the elements I'5 and I6, the over-all impedance of the `network I4 may be .determinedat any frequency and a curve vplotted illustrating this impedance as afunction of frequency.

Referring to Fig. 3 of thedrawing I have shown a curve obtained in theabove described manner illustrating, by way of example, thecharacteristic impedance curve .determined from data taken on the testcircuit referred to above. In this circuit the impedance of themodulated load was 5,00G ohms and the reactor Il ,presented an impedanceof approximately 10,000 ohms capacitive reactance to signal currentshaving a frequency of Cil 100,000 cycles. From this data the necessaryvalues of elements I5 and I5 were determined at .032 henries and 20,000ohms respectively. With the impedance values of the elements I5, I0 andI'I thus determined the curve C was plotted.

It will be observed from curve C that the overall impedance of thenetwork is very high in the low frequency portion of the signalfrequency range due to the effect of the reactor I'I, and that itdecreases to a minimum value of 10,000 resistive ohms at approximately100 kilocycles due to the effect of the series inductive reactance I5 inthe equivalent series circuit on the capacitive reactance of the reactorII. As the frequency is increased above 100 kilocycles the inductivereactance of 15 increases with the frequency to an exceedingly highvalue and the capacitive reactance of the reactor Iii approaches a zerovalue thereby leaving the resistance I6 as the effective couplingimpedance. This effect is clearly illustrated by curve C which showsthat the over-all impedance of the test circuit considered approached20,000 ohms, or a value approximately equal to the value of theresistance.

In practice it was found that the distributed capacity of the coil I5affected slightly the overall impe-dance frequency characteristic curveof the coupling network. The test values obtained indicated that theactual impedance values over certain portions of the frequency range ofthe network were slightly above the calculated values. The effect ofthis capacity on the modulator stage is effectively cancelled by theinductance of the coil i8 in the same manner that the inductanceeffectively equalizes the stray capacity of the carrier power amplifiercircuit.

From the foregoing analysis it will be observed that at the lowerfrequencies the impedance of the iron core reactor I1 predominates andthat in the upper portion of the frequency range the parallel-connectedresistance I6 and inductance I5 are effective to supply the necessaryimpedance, It will further be observed that in the upper portion of thefrequency range the impedance is largely determined by the resistance i0and that, therefore, this resistance should bear a predeterminedrelation to the impedance of the modulated load. For best operatingresults the value of the resistance I6 should be approximately fourtimes that of the load impedance and the impedance I 5 should have areactance equal in magnitude to that of the resistance at the lowerarbitrary limit of impedance of the reactor I'I. In order to main theeffective series impedance I5', Iii high for a considerable range abovethe natural resonant frequency of the inductance I5, the inductance I5should have a very low distributed capacity. This may be secured byconstructing the inductance I5 of a plurality of series-connecteduniversal wound thin disc coils each having a very low distributedcapacity.

From the foregoing description it will be observed that by employing theelements I1 and It connected in the manner illustrated and proportionedin value with respect to the modulated load impedance in accordance withthe principles outlined above, an over-all impedance is obtained whichinsures satisfactory modulation over an extremely wide band' offrequencies. It will further be seen that my improved coupling impedancenetwork is particularly useful in a high power transmitter installationwherein relatively high direct currents flow thru the coupling impedancenetwork elements and extremely high voltages exist between the terminalsof the network. Thus, it will be seen that my improved couplingarrangement is particularly useful in those applications where it is noteconomically practical to use a large non-inductive resistance as acoupling element.

Although I have described my invention as being particularly useful inconnection with television transmitters, it will be understood that itis susceptible of being employed in other high frequency applications aswell. Thus, for example, my improved coupling network may be employed inany installation wherein it is desired to couple signal currents havingfrequencies varying over an extremely wide range to a utilizing circuitof any type.

While I have described what I consider to be the preferred embodiment ofmy invention, it will of course be understood that I do not wish to belimited thereto since many modications in the circuit may be made,V andI contemplate by the appended claims to cover all such modifications asfall within the true spirit and scope of my invention.

What I claim as new and desire to secure by Letters Patent of the UnitedStates is:

1. In combination, a source of signal currents having desiredfrequencies extending over a'wide range, a utilizing circuit, a couplingimpedance network for impressing said signal currents on said utilizingcircuit, said network including an inductance coil having a highinductive reactance at low frequencies within said range and a lowimpedance below a predetermined value at high frequencies within saidrange, an inductance coil in series with said first-named inductancecoil having high reactance at said high frequencies, and a resistance inshunt with said second-named inductance coil of such value that theimpedance of said coupling impedance network is increased by reason ofsaid shunt resistance to a value above said predetermined value of saidErst-named inductance coil at said high frequencies.

2. In combination, a source of signal currents having desiredfrequencies extending over a wide range, a utilizing circuit, a couplingimpedance network for impressing said signal currents on said utilizingcircuit, said network including an inductance coil having a highinductive reactance at low frequencies within said range and `a lowimpedance below a predetermined value at high frequencies within saidrange, an inductance having a low distributed capacity connected inseries with said first-named inductance, said secondnamed inductancehaving high reactance at said high frequencies, and a resistance inshunt with said second-named inductance of such value relative to theimpedance thereof that the impedance of said network is increased abovesaid predetermined value at said high frequencies,

3. In combination, a source of signal currents having desiredfrequencies extending over a wide range, a utilizing circuit, a couplingimpedance network for impressing said signal currents on said utilizingcircuit, said network including an inductance coil having a highinductive reactance at low frequencies within said range and a lowcapacitive impedance below a predetermined value at high frequencieswithin said range, and means including an inductance having a lowdistributed capacity connected in series with said first-namedinductance and a resistance connected in shunt with said last-namedinductance for maintaining the impedance of said network above saidpredetermined value at said high frequencies, the magnitude of theimpedance of said last-:named inductance being such as to `neutralizethe rcapacitive'impedance of said first-named inductance at thatfrequency at which theimpedance of fsaid'rst-named inductance equalssaid predetermined value, and-said resistance `having aivaluesubstantially equal to said magnitude of the impedance of saidsecond-named inductance at said `last-named frequency.

4. In combination, a source `of signal currents having desiredfrequencies extending-over a wide range,.azutilizing circuit, a couplingimpedance network-*connected toimpress said signal currents on `said:utilizing circuit, said network including an inductance coil having lowdistributed capacity shunted by a resistance and connected in seriesWith an inductance coil having a high distributed capacity, theimpedance of said coils in series being increased at a certain frequencyin said range by saidiresistance.

5. In'combination, a source of signal currents having desiredifrequencies-extending over a wide range, a utilizing circuit, acoupling impedance network connected to impress said signal currents on`said utilizingpcircuit, said network including an inductance coilhaving low distributed capacity yshunted by a resistance and connectedin serieszwithan inductance coil having a high distributed capacitywhich introduces a capacitive reactance into said network at frequenciesabove aipredetermined'value within said range, said resistance being -ofsuch value asto increase the impedance between `the outer terminals ofsaid coils.

6. VIn combination, -a source of signal currents having desiredfrequencies extending over a wide range, a utilizing circuit, a couplingimpedance network connected to impress said signal currents lon saidutilizing circuit, said network including aninductance coil having lowdistributed capacity shunted by a resistance yand connected in serieswith an inductance coil having a high distributed lcapacity, saidAsecond-named inductance :having :an inductive reactance at lowfrequencies within said yrange and a capacitive reactance below apredetermined value at high frequencies within said range, 'themagnitude of the impedance of `said iiirst-named inductance being suchas to --neutralize the capacitive impedance of said -second-namedinductance at that frequency at vwhich the impedance of saidlsecondnamed inductance equals said predetermined value, and saidresistance having a value substantially greater than the magnitude ofthe impedance of said second-named inductance at Vsaid last-namedfrequency, whereby the impedance of said network is vmaintained abovesaid predeterminedminimum value at all frequencies within said range.

J7. In combination, a .source of signal currents having-desiredfrequencies-extending over a wide range, a utilizing circuit and acoupling impedance network rfor impressing said signal currents on saidutilizing circuit, said network including an air corel-reactor `shuntedby a resistance and connectedin .series with an iron core reactor, saidresistance being -of such value as to increase the impedance between theouter terminals of said network.

v8. In combination, a source of signal currents having desiredfrequencies extending over a wide range, ya utilizing circuit, acoupling impedance network for impressing said signal currents on said.utilizing circuit, said network including an inductance coil havingahigh inductive reactance at low-frequencies Within said range and a lowcapacitive impedance belowa predetermined value at1high frequencies-within said range, and means for. maintaining .the impedance of saidnetwork above ya predetermined value at said high frequencies, saidlast-named means including a plurality .of series-connected inductancecoils each having a low distributed capacity connected in series withsaid first-named inductance and a resistance connected in parallel withsaid lastnamed series connected inductance coils, said resistance havingYa value at least two times the magnitude of said predetermined value,and said series-connected Vinductance coils having an impedance valueequal to said resistance at that frequency at which the impedance ofsaid rst named inductance equals said predetermined value.

9. Incombination, a source of signal currents having desiredfrequencies-extending over a wide range, fa .utilizing circuit, acoupling impedance network .for impressing said signal currents on saidutilizing circuit, said network including an inductance coil having ahigh inductive reactance at vlow frequencies within said range and a lowimpedance below a predetermined value at high frequencies within saidrange, and means including an inductance havinga low distributedcapacity connected in series with said first-named inductance and aresistance connected in shunt .withsaid last-named inductance formaintaining vthe impedance of said network above said predeterminedAvalue at said high frequencies, said resistance having a value at leasttwo times the magnitude .of said predetermined value, and said secondnamed inductance having an impedance .value equal vto said resistance atthat frequency where the impedance .of -said first named inductanceequals .said predetermined value.

VCrEORGE W. FYLER.

